(1) Field of the Invention
This invention relates to a method for preparing ethylene glycol and/or propylene glycol. More particularly, it relates to a method for preparing ethylene glycol and/or propylene glycol with a catalyst comprising a carboxylic acid and a carboxylic acid salt or a metal salt of formic acid alone.
(2) Description of the Prior Art
Conventionally, as methods for preparing lower alkylene glycols, there is widely employed in industry methods for conducting a hydration reaction of lower alkylene oxides as epoxy compounds with water without catalysts or with acid catalysts, for example, mineral acids such as sulfuric acid [for example, refer to S. A. Miller, Ethylene and its Industrial Derivatives, page 588-594 (Ernest Benn Ltd., 1969)].
This process, however, requires to depress as much as possible the formation of by-products including diethylene glycol, triethylene glycol, dipropylene glycol and tripropylene glycol which are commercially in a small demand. Thus it is forced to employ a large excess of water, the quantity of which is about 10 to about 30 miles per 1 mole of the lower alkylene oxide, and the desired lower alkylene glycols were obtained only as dilute aqueous solutions.
Therefore, the process has a disadvantage that, after the end of hydration reaction, a large amount of energy is consumed in order to concentrate, dehydrate and fractionate the reaction mixture and thus it becomes economically unfavorable unfavorable.
Base catalysts have the effect of accelerating the hydration reaction, however, they generally increase the quantity of by-products as compared with the above described reactions without catalysts or with acid catalysts. Since the selectivity of desired lower alkylene glycols is apt to decrease (for example, refer to above Ethylene and its Industrial Derivatives, page 594, lines 16-24), the hydration reaction with the base catalysts has not been applied in industry.
On the other hand, a method has recently been proposed to overcome the drawbacks of the above described manufacturing process, wherein lower alkylene oxides are hydrated in high concentrations with equivalent amounts of water in the presence of carbon dioxide and with tetraalkyl ammonium salts or quarternary phosphonium salts as a catalyst.
The above described method of conducting the hydration reaction in the presence of carbon dioxide has not yet satisfactorily developed in view of the catalyst and cost performance. It has also a disadvantage that the method uses a large amount of carbon dioxide (generally employed in the form of gas), thus requires relatively high reaction pressure as compared with the above described methods of industrial production which are currently in a wide application, and leads to a high construction cost of the plant.
Therefore, development of a novel hydration reaction technology is now earnestly desired which enables the hydration reaction of lower alkylene oxides in high concentrations and also can prepare lower alkylene glycols more favorably with a high selectivity and at a high yield.